In recent years, with increased traffic safety consciousness, air bags are used to ensure safety of passengers in the occurrence of automobile accident. Its effectiveness is recognized with developments of various airbags, and air bags are rapidly put to practical use.
The airbag receives passengers moving by a reaction of collision and absorbs its impact to protect passengers by being expanded and deployed in an extremely short time upon collision of vehicles. On the other hand, when automobiles are used normally, the airbag is folded and stored since it is not used. If the weight of the airbag can be reduced, vehicle weight can be reduced. If the airbag can be compactly folded, the space to store the airbag can be reduced and, hence, an interior space can be widened. Therefore, concerning the base fabric constituting an airbag cushion, requirements for lightweight properties and compactness are increased to increase an interior space and improve fuel economy.
On the other hand, concerning a gas generator (inflator) which has a large weight ratio among parts constituting the airbag, requirements for lightweight properties and compactness are increased, and a lightweight and low-cost pyro-inflator is becoming a mainstream in place of a cold inflator using compressed gas and a high-pressure gas vessel and a hybrid inflator using a gunpowder in combination with compressed gas. The pyro-inflator principally generates a gas with the gunpowder, and therefore a gas generated has a high-temperature and is rapidly thermally expanded, and therefore, high heat resistance and mechanical properties are required of the airbag base fabric.
Conventionally, as a means to enhance heat resistance of the airbag base fabric and achieve lightweight properties and compactness, a coated base fabric having a thermosetting silicone resin applied thereto, and a coated base fabric having a film bonded thereto are proposed. However, since the coated base fabric uses an expensive resin, it has a problem that cost is high compared to the base fabric to which a resin is not applied (hereinafter, may be referred to as a non-coated).
Thus, to solve such problems, development of a non-coated base fabric is performed. For example, an airbag base fabric is disclosed which is excellent in compactness while retaining adequate low air permeability as a non-coated base fabric and mechanical properties by shaping a single fiber cross section of a synthetic fiber multifilament constituting a non-coated base fabric into a flat shape, and drawing up cross sections of single fibers (refer to Japanese Patent Laid-Open Publication No. 2003-171841). Moreover, a manufacturing method of an airbag base fabric that can achieve further low air permeability by applying shrinking to an textile woven using the threads in which single fiber cross sections are flat, is disclosed (refer to Japanese Patent Laid-Open Publication No. H10-37039).
However, when producing an airbag cushion using the airbag base fabric, particularly in applications of airbags for a driver's seat and a passenger's seat, after sewing a cushion in the form of a bag, the cushion is reversed to arrange a sewn portion on an inner side, and thereafter the cushion is folded, and on this occasion, the base fabric is kneaded. Naturally, a base fabric using flat threads can be used for a conventional reversing work or folding work without problems. However, by a recent increasing requirement for compactness, to reduce a cushion volume after folding, a folding nature is given by a stronger force, or to be housed in a very narrow package, the cushion is kneaded by a stronger force. In this time, arrangement of the single fibers drawn up in the base fabric portion constituting a cushion is disordered, and there is a concern that air permeability may be increased, and therefore an airbag base fabric composed of threads having a circular cross section which is low in change of air permeability with respect to strong kneading, which is thin and compact, is required.
Further, for example, to provide a non-coated base fabric having low air permeability and is flexible, lightweight and inexpensive, widths of threads of warp threads and weft threads constituting the base fabric are increased (refer to Japanese Patent Laid-Open Publication No. 2005-105445), and an airbag base fabric which is lightweight and excellent in package ability while retaining mechanical properties and flame retardancy by setting relative viscosity of synthetic fiber multifilament constituting, an amount of amino-terminals and characteristics of decomposed woven threads to specific ranges is developed (refer to Japanese Patent Laid-Open Publication No. H09-279437), and an airbag textile in which to reduce the weight of an inflator itself by efficiently using an inflator gas, characteristics of decomposed threads of polyamide fibers constituting the textile, and air permeability of the textile and a state of thread width are set to specific ranges is developed (refer to International Publication WO 2011/055562).
However, in recent years, the number of sites equipped with an airbag increases such as a knee airbag that protects knees, a rear airbag that protects passengers in a rear seat from rear-end collision, and an airbag for a pedestrian that protects a pedestrian, and requirements for lightweight properties and small thickness of the airbag base fabric are increasing, but the airbag base fabric satisfying these requirements has not yet been developed.
It could therefore be helpful to provide an airbag base fabric which is lightweight and thin while retaining mechanical properties and low air permeability required of an airbag base fabric, and a method of manufacturing the airbag base fabric.